1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus and, more particularly, to a construction of a separation feeding unit for separating and feeding sheets one by one to an image forming unit.
2. Description of the Related Art
Hitherto, an image forming apparatus such as copying apparatus, printer, or facsimile has a sheet feeding apparatus for feeding a sheet to an image forming unit, and the sheet feeding apparatus has a sheet separation feeding unit for separating and feeding the sheets one by one.
As such a sheet separation feeding unit, there is a sheet separation feeding unit having a feed roller and a retard roller (separating roller) which is come into pressure contact with the feed roller and to which a rotating force in the direction opposite to a sheet feeding direction is applied through a torque limiter. In the sheet separation feeding unit of such a retard separating system, the sheets are separated one by one by the operation of the torque limiter.
FIG. 10 is a diagram illustrating a construction of the sheet feeding apparatus in the related art having the sheet separation feeding unit of such a retard separating system. FIG. 10 illustrates a cassette 200 in which sheets S are stored (supported), a side wall 203 on the downstream side in the sheet feeding direction of the cassette 200, and a middle plate 201 provided for the cassette 200 so as to be movable in the vertical direction. The sheets S have been stacked on the middle plate 201.
A pickup roller 129 feeds a top sheet Sa among the sheets S enclosed in the cassette 200. A feed roller 130 is made of, for example, urethane. A retard roller 131 is made of, for example, EPDM and is come into pressure contact with the feed roller 130 by a spring (not shown). When the retard roller 131 is come into pressure contact with the feed roller 130, a separation nip portion N is formed between the retard roller 131 and the feed roller 130 and a surface portion of the retard roller 131 which is in contact with the feed roller 130 is compressed.
The top sheet Sa fed out by the pickup roller 129 is conveyed to the separation nip portion N formed by the feed roller 130 and the retard roller 131.
A sheet feeding frame 301 holds a spring (not shown) adapted to allow the retard roller 131 to be come into pressure contact with the feed roller 130. A conveying path 303 is formed by a conveying guide 302 provided between the separation nip portion N and a pair of conveying rollers 305a and 305b. A detecting sensor flag 304 detects a passage of the sheet S.
In the sheet feeding apparatus in the related art constructed as mentioned above, in the case of feeding the sheets, first, the pickup roller 129 rotates and the top sheet Sa supported in the cassette 200 is conveyed to the separation nip portion N.
As a sheet Sa fed to the separation nip portion N, in the case where only one sheet has been conveyed to the separation nip portion N, the retard roller 131 is driven by the sheet Sa by the operation of the torque limiter as illustrated in FIG. 11. Thus, the sheet Sa passes along the conveying path 303.
If a plurality of sheets has been conveyed to the separation nip portion N, the retard roller 131 is rotated in the direction opposite to the sheet feeding direction of the feed roller 130 by the operation of the torque limiter as illustrated in FIG. 12 without being driven by the feed roller 130. Since the retard roller 131 is rotated in the direction opposite to the sheet feeding direction of the feed roller 130 as mentioned above, only one sheet Sa which is in contact with the feed roller 130 is conveyed and the other sheets are returned to the upstream side in the sheet feeding direction by the retard roller 131.
In such a sheet feeding apparatus in the related art, there is a case where the surface of the retard roller 131 is scraped due to deterioration in durability or the surface of the retard roller 131 is charged by a slide friction between the retard roller 131 and the conveyed sheet S. When the surface is charged, a foreign matter (mainly, paper powder generated from the sheet) is deposited onto the surface of the retard roller 131 and a coefficient of friction of the surface of the retard roller 131 decreases.
Ordinarily, the retard roller 131 has halted by a friction caused by the pressure contact with the feed roller 130 when the feed roller 130 has been stopped. When the feed roller 130 starts to rotate, the retard roller 131 rotates in the sheet conveying direction synchronously with the rotation of the feed roller 130. However, when the coefficient of friction of the surface decreases, the retard roller 131 starts to rotate in such a direction as to return the sheet in the direction opposite to the sheet feeding direction of the feed roller 130 irrespective of the stop/rotation of the feed roller 130.
That is, inherently, the retard roller 131 rotates in the reverse direction in the case where a plurality of sheets has been conveyed to the separation nip portion N as mentioned above. However, when the coefficient of friction of the surface decreases, even in the case of feeding one sheet, the retard roller 131 rotates in the direction opposite to the rotating direction adapted to feed the sheet.
When the retard roller 131 rotates in the direction opposite to the rotating direction adapted to feed the sheet, the following problems occur. When the sheet Sa fed out by the pickup roller 129 collides with the retard roller 131 as illustrated in FIG. 13, a sheet front edge is rolled in by the retard roller 131 as illustrated in FIG. 14 and there is a risk of occurrence of a defective feeding state where the front edge is bent.
Further, if the sheet S is curled toward the retard roller 131 side, the defective feeding of the bent front edge is more liable to occur. Since the sheet front edge collides with the surface of the retard roller 131, a scratch occurs on the surface of the retard roller 131 and the defective feeding is further liable to occur.
To prevent such a drawback, in the sheet feeding apparatus having the sheet separation feeding unit of the retard separating system, a guide is provided at a front edge in the sheet feeding direction of the cassette. The sheet fed out by the feed roller 130 is guided to the separation nip portion N by the guide. By guiding the sheet toward the separation nip portion N by the guide, it is prevented that the sheet front edge is rolled in by the retard roller 131.
As such a guide, for example, there is a guide arranged in such a manner that one end is supported to a fixing portion of a sheet conveying path and the other end is extended to a position near the separation nip portion from an almost tangential direction of an outer surface of the retard roller 131. Such a construction has been disclosed in Japanese Patent Application Laid-Open No. H05-338837.
FIG. 15 is a diagram illustrating a construction of the sheet feeding apparatus in the related art having such a guide. A sheet guide 135 is made of a thin elastic member and is in elastically come into contact with the outer surface of the retard roller 131 from its almost tangential direction.
By allowing the sheet guide 135 to be come into contact with the outer surface of the retard roller, the sheet front edge can sufficiently reach the position near the separation nip portion N without colliding with the retard roller 131. Thus, even when the retard roller 131 is rotated in the direction opposite to the sheet feeding direction, it is possible to prevent such an inconvenience that the sheet is rolled in by the retard roller 131 and the occurrence of the damage on the surface of the retard roller 131.
In the sheet feeding apparatus in the related art, as already mentioned above, the surface of the retard roller 131 is scraped due to the deterioration in durability or the surface of the retard roller 131 is charged by the slide friction with the conveyed sheet S.
If the foreign substance such as a paper powder is deposited onto the surface of the retard roller 131 by a surface charging and the coefficient of friction of the surface of the retard roller 131 decreases, the rotation in the direction opposite to the sheet feeding direction of the retard roller 131 as mentioned above is caused. Particularly, in the case of using such a sheet that the paper powder is liable to be generated, the coefficient of friction decreases rapidly due to a large quantity of paper powder deposited on the surface and the rotation in the reverse direction of the retard roller 131 is caused early. Thus, the sheets cannot be stably separated and fed.
The sheet guide 135 illustrated in FIG. 15 can be constructed in such a manner that even in the case where the retard roller 131 is rotated in the reverse direction, the front edge of the sheet is not come into contact with the retard roller 131 as much as possible. However, the rotation in the reverse direction of the retard roller 131 cannot be prevented. Therefore, even if the sheet guide 135 has been arranged, there is a large risk that the front edge of the sheet is come into contact with the retard roller 131 which is reversely rotated and the foregoing problem occurs.